The present invention relates to fiber-optic communications networks, and more particularly, to optical amplifiers with transient control capabilities for use in optical communications networks.
Fiber-optic networks are used to support voice and data communications. In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber.
Optical amplifiers are used in fiber-optic networks to amplify optical signals. For example, optical amplifiers may be used to amplify optical data signals that have been subject to attenuation over fiber-optic links. A typical amplifier may include erbium-doped fiber coils that are pumped with diode lasers. Raman amplifiers have also been investigated. Discrete Raman amplifiers may use coils of dispersion-compensating fiber to provide Raman gain. Distributed Raman amplifiers provide gain in the transmission fiber spans that are used to carry optical data signals between network nodes.
Sometimes channels in a communications link may be abruptly added or dropped due to a network reconfiguration. Channels may sometimes be dropped due to accidental fiber cuts. When the number of channels carried by a link changes abruptly, the total signal power being transported over the link changes suddenly. This can adversely affect the performance of distributed or discrete amplifier equipment in the link.
For example, if an erbium-doped amplifier is pumped at a constant power, these sudden changes in signal power will result in transient effects in the gain of the amplifier. Gain transients in the amplifier may cause fluctuations in the power of the output signals at the output of the amplifier. Output signals that are too weak may be difficult to detect at a receiver. Output signals that are too strong may give rise to nonlinear optical effects.
Abrupt changes in the signal power on a link may also cause undesirable transient effects in the gain of other rare-earth-doped fiber amplifiers or Raman amplifiers.
It is an object of the present invention to provide optical amplifier systems with transient control capabilities for use in fiber-optic communications networks.
It is also an object of the present invention to provide optical amplifiers having optical delay lines.
It is also an object of the present invention to provide optical amplifiers that can measure optical delays associated with different installed midstage components and can use information on the measured delays in controlling gain transients.
These and other objects of the invention are accomplished in accordance with the present invention by providing optical amplifiers for communications networks such as those based on wavelength-division-multiplexing optical communications links that support multiple channels operating at different wavelengths. Optical amplifier equipment may be used to amplify optical signals on a communications link. Optical amplifiers may be based on distributed or discrete Raman amplifiers, rare-earth-doped fiber amplifiers such as erbium-doped fiber amplifiers, or any other suitable amplifiers.
The optical signals on the link may be monitored using optical monitors. Amplifiers may be provided in which input signals are monitored using input taps and output signals are monitored using output taps. Gain transients may be suppressed using feed-forward and feedback control techniques. Optical delay lines may be used to enhance the gain transient suppression capabilities of the amplifiers.
Optical amplifiers may be provided that have replaceable midstage modules. The midstage modules may, for example, be dispersion compensation modules based on coils of different lengths of dispersion-compensating fiber. The optical amplifiers may automatically detect the length of the dispersion-compensating fiber so that amount of optical delay associated with that fiber length may be taken into account when suppressing gain transients during operation of the amplifier.
Further features of the invention and its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.